Hmmm... Well, there's the story of Nightmare Moon. That's probably the one I could do the best at.

[Yes, because you were there for about the last half of the story, Twi.]

It started a long, long time ago, over a thousand years in fact. In the land of Equestria where I come from. Equestria was ruled by two princesses, the elder one who governed the sun and gave the ponies who lived with her daytime, and the younger one who controlled the moon and gave them night. The two sisters ruled in harmony for many years.